Tamper evident screw closures

ABSTRACT

A screw closure for a container includes a tamper indicating ring, a frangible link connecting the ring to the remainder of the screw closure, and a flexible anti-backoff pawl having a root end attached to the tamper-indicating ring. A distal end of the pawl co-operates with a rack tooth formed on the container so as to allow relative rotation between the ring and container in the closure screwing-on direction and so as to resist relative rotation between the ring and container in the closure unscrewing direction. The distal end of the pawl is connected to the ring by a flexible link. Alternatively the pawl is supported in the unscrewing direction by a guard stop or the distal end has an enlarged cross-section whereby it is stiffened to resist bending, and/or the ring is protected by radially projecting covers.

Closure caps are known which include a so-called tamper indicating ring, which is broken off from the remainder of the cap the first time that the cap is removed from a filled container. An intact ring provides assurance that the container contents have not been interfered with after application of the cap to the container for the first time.

One form of tamper indicating ring widely used in injection moulded plastics screw caps is attached to the remainder of the cap by a set of frangible links. The ring is provided on its inner circumference with a series of flexible blades which act as “anti-backoff” ratchet pawls, in co-operation with a set of teeth, usually moulded into the container neck, which act as the ratchet rack. The blades extend radially inwardly from the ring and slope obliquely rearwardly from root to tip in the screwing-on direction of the cap. Thus when the cap is viewed from below, for a right hand threaded cap, there is an acute included angle between the clockwise side of the blade and the ring. Correspondingly, the container rack teeth each have a (clockwise-facing, when the container is viewed from above) flank lying substantially in a radial plane, to provide a surface which faces forwardly in the cap screwing-on direction; and a further flank which slopes from the tooth tip to the base of the tooth. Therefore when the cap is screwed onto the container towards its final position, the blade tips ride up along, and are cammed outwardly by, the teeth sloping flanks and then spring inward behind the teeth radial flanks. This ratcheting process can continue until the cap is fully tightened. The frangible links are sufficiently strong to move the ring in unison with the remainder of the cap as a screwing-on torque is applied, pulling the flexible blades past the rack teeth in succession. When the cap is unscrewed, the blade tips jam against the teeth radial flanks and hold the tamper-indicating ring stationary on the container. Continued unscrewing of the cap therefore breaks the frangible links. When the cap is removed, the ring remains behind on the container. A user can therefore easily tell from a detached or missing ring that the container has already been opened.

However this form of tamper indicating ring can be defeated by a technique which involves gripping both the ring and the remainder of the cap in a way which ensures that these two components move in unison in the unscrewing direction, thereby relieving any strain on the frangible links. A sufficiently high torque can be applied to the ring in this way, to cause the flexible blades to bend abnormally and opposite to their normal flexing direction, so that their clockwise sides when viewed from below, make an acute angle with the ring, at least at the blade tips. Hence the blades can be forcibly snapped past the neck teeth (perhaps distorting the container radially inwardly and/or stretching and expanding the ring), and the cap can be unscrewed with the frangible links left intact. This forcing process may bend and break the blade tips, but the blades may be left sufficiently intact so that when the cap is re-applied to the container, they are bent back towards their proper positions and may even serve to hold the ring stationary the next time the cap is unscrewed. This provides a false indication that the cap has not previously been opened. Importantly, the forcing process leaves the frangible links intact and so requires very close and careful inspection of the blade tips in order to be detected, if detectable at all.

A variant of this tamper indicating ring is known which is less vulnerable to the above-described forcing process, in that the pawl members on the ring are formed as solid, substantially triangular teeth, each having a radial flank for lockable co-operation with the container rack teeth radial flanks, and a sloping flank for co-operation with the container rack teeth sloping flanks. The ring itself flexes radially outwardly to allow the ring tooth and rack tooth sloping flanks to slide past each other as the container cap is screwed on. As the cap is unscrewed, the ring tooth and rack tooth radial flanks lock together to hold the ring stationary and break the frangible links. The solid ring teeth are too stiff to bend back on themselves under attempted forced unscrewing. Instead, the forcing tool tends to slip relative to the ring and sufficient movement is generated between the ring and the remainder of the cap to cause the frangible links to break. However, with this cap design, in normal use quite high torques are applied to the frangible links as the cap is screwed on. Therefore there is a danger that the links may break to give a false positive tamper indication the first time that the cap is screwed on, and/or a danger that the frangible links must be made so strong that the cap is difficult to remove for the first time. Further improvements in such tamper indicating rings are therefore desirable.

JP H07-291318 (Ishizuka Glass) discloses a synthetic resin cap 1 having a pilfer proof skirt portion 5 retained on a mouth portion 100 of a glass bottle by wing pieces 11. V-shaped claw portions 12 connect the inner surface of the wing piece 11 and the skirt portion 5. The V-shaped claw portions allow the wing pieces to move outwardly past jaws 103 on the container neck and then elastically bias the wing pieces 11 into engagement with the bottom surface 104 on the container neck below jaws 103, as the synthetic resin cap 1 is screwed onto the glass bottle. The V-shaped claw portions 12 are thin and may therefore break when the wing pieces 11 are bent abnormally in the reverse direction.

JP H08-198287 (Crown Cork Japan) concerns a tamper evident (TE) cap provided with a TE band having a thickened upper portion and a relatively thinner lower portion. The lower portion is provided with ratchet pieces made from first and second pieces 17 a and 17 b consolidated with the TE band to form a hollow wedge shape. Upper end portions of the pieces 17, 17 b are connected to the thick upper portion 10 of the TE band. The arrangement is said to allow the TE cap to be easily screwed on without preventing a weakened portion of the TE band from breaking and so allows the TE band to be reliably separated from the cap body during lid opening. There is no disclosure or suggestion that the projecting pieces 17 a constrain the projecting pieces 17 b against abnormal bending in the event that a forcing tool is attempted to be used, whereby the forcing tool will slip on the TE band.

JP H09-077104 (Nippon Glass KK) in FIG. 9 shows locking portions 13, 13A for a tamper evident band 7. Parts (A) to (D) of FIG. 9 show stages in the folding of the locking portion 13A as initially moulded, so as to form a completed locking portion 13. The completed locking portion 13 is able to fold flat against the inner wall of the screw cap 1 as it is screwed onto a container neck. Part (E) of FIG. 9 shows an alternative embodiment in which the locking portion has a W-shaped profile, and again is able to fold flat as the cap 1 is screwed on. Again, in no case is there any disclosure or suggestion that the locking portion resists abnormal bending so that attempts to use a forcing tool to turn the tamper evident band 7 as the cap 1 is unscrewed will result in the tool slipping and the frangible links 11 of the tamper evident band 7 breaking.

Accordingly the present invention provides a screw closure for a container, as defined in claim 1. If an attempt is made to force the tamper indicating ring to rotate together with the remainder of the screw closure in the unscrewing direction, the flexible link constrains the anti-backoff pawl and increases its resistance to being bent in an abnormal direction when it encounters the rack tooth on the container. The forcing tool is therefore more likely to slip relative to the tamper indicating ring, causing the frangible links to break as intended. At the same time, both the flexible anti-backoff pawl and the flexible link can be made to be relatively easily deformable in comparison to a solid tooth on the tamper indicating ring, whereby screwing on of the closure is easier and imposes lower stresses on the frangible link.

The flexible link may be made sufficiently thin whereby it readily undergoes buckling collapse during screwing on of the closure. Additionally or alternatively it may be provided with one or more thinned sections along its length, to form more flexible sections or “living hinges”, for the same purpose. A flexible link which buckles easily will offer less mechanical resistance to screwing the closure onto or into the container and impose a lower shear load upon the frangible link. The flexible anti-backoff pawl may have an elongated blade-like shape. The tip region of the blade may be tapered, chamfered or faceted to engage in a recess formed between the base of a rack tooth and the adjacent surface of the container neck. The flexible link may be as broad as the blade-like shape of the anti-backoff pawl, so as to be strong enough to constrain abnormal (reverse) bending of the anti-backoff pawl without breaking, but yet be flexible enough to undergo buckling collapse relatively easily, so as not to hinder the normal movement of the anti-backoff pawl excessively during screwing-on or screwing-in of the screw closure.

Additionally or alternatively, the flexible link may be curved or bent along its length in the relaxed state. This again encourages the flexible link to collapse or deform under lengthwise compression, in order to allow the flexible anti-backoff pawl to flex and more easily move past the rack tooth in the closure screwing-on/in direction. The resulting lower screwing on torque which then needs to be applied to the remainder of the closure means that a weaker frangible link can be used. This ensures that the closure is easier to remove on the first occasion.

For example the flexible link may have a bowed shape. The centre of the bowed shape may face towards or away from the root end of the flexible anti-backoff pawl. On unscrewing the closure for the first time, the flexible anti-backoff pawl encounters the rack tooth and begins to bend radially inwards of the closure. This tends to straighten the bowed flexible link and allows the flexible anti-backoff pawl to engage more deeply behind the rack tooth on the container.

The flexible link may be joined to the flexible anti-backoff pawl at or near to the distal end thereof, so as to provide favourable mechanical advantage and favourable control against excessive bending of the flexible anti-backoff pawl under an unscrewing torque.

Although flexible so as to allow movement past the rack tooth in the screwing-on direction, the anti-backoff pawl may be sufficiently stiff so as to act as a strut which is resistant to buckling collapse. It thereby provides a high resistance to movement of the tamper indicating ring in the unscrewing direction, when the distal end is wedged against the container rack tooth. The flexible anti-backoff pawl thus may be flexibly attached to the tamper indicating ring, for example comprising a thinned section, or even a living hinge, at or near to its root end. This allows the necessary normal flexing of the pawl so that it can move past the rack teeth as the closure is screwed onto the container. Either the flexible attachment at the root end, or the flexible link, or both, may provide the flexible anti-backoff pawl with resilient bias so that it engages behind and jams against the rack tooth when the closure is moved in the unscrewing direction.

The tamper indicating ring may be interrupted to form segments, with ends of adjacent segments connected in series by the flexible anti-backoff pawl and the flexible link.

Provision of a flexible link as described above is not the only solution to the problem of improving the resistance to abnormal bending of the anti-backoff pawl if forced unscrewing of the tamper indicating ring is attempted. As one alternative, the present invention provides a screw closure for a container, as defined in claim 10. Thus the guard stop may be positioned behind the anti-backoff pawl in the closure unscrewing direction.

The guard stop may be thinner than the width of the tamper indicating ring in the closure axial direction and positioned adjacent to the upper edge of the tamper indicating ring; or the guard stop may be thinner than the width of the tamper indicating ring in the closure radial direction and positioned adjacent to the inner or outer edge of the tamper indicating ring; in each case so as to provide a clearance space for passage of the rack tooth formed on the container.

Alternatively, the guard stop may comprise an extension of an annular side wall of the screw closure positioned opposite to the distal end of the anti-backoff pawl. The ratchet teeth may be spaced from the container neck, e.g. on the container body, to define an annular gap between the container neck and the ratchet teeth, into which the screw closure's annular side wall extension extends when the closure is fully tightened. The annular side wall extension and the container neck form a relatively rigid structure which constrains abnormal (backwards) bending of the anti-backoff pawl.

As another alternative solution to the problem of improving the resistance to abnormal bending of the anti-backoff pawl if forced unscrewing of the tamper indicating ring is attempted, the present invention provides a screw closure for a container, as defined in claim 12. For example, the flexible anti-backoff pawl may progressively increase in cross-sectional thickness along its length from the root end towards the distal end. The flexible anti-backoff pawl may be flexibly attached to the tamper indicating ring, for example comprising a thinned section, at or near to its root end.

The distal end of the flexible anti-backoff pawl may comprise a first, substantially flat face configured for substantially face-to-face engagement with a generally radially or vertically extending face of the rack tooth on the container, and optionally a second face for engagement with the container (on the part of the container onto which the closure is being screwed, typically a neck), adjacent to the rack tooth.

The closure may further comprise a flexible link as described above, connecting the flexible anti-backoff pawl and the tamper indicating ring.

As yet another alternative solution to the problem of improving the resistance to abnormal bending of the anti-backoff pawl if forced unscrewing of the tamper indicating ring is attempted, the screw closure may further comprise a cover overlying at least a part of the outer, radially outwardly facing, circumference of the tamper indicating ring. The cover thereby prevents access to the full circumferential extent of the tamper indicating ring, making driving engagement with the tamper indicating ring for forced bending and disengagement of the flexible anti-backoff pawl more difficult.

The cover may leave at least one section of the tamper indicating ring exposed and visible, so that its presence or absence can readily be seen when the closure is screwed onto or into the container. The cover may comprise one or more sections. The or each section may stand out radially from the tamper indicating ring to an extent that at least 50% of the ring lies between tangents to the ring and the outer portion of the envelope of the or each cover section. In this way, the ring may be provided with at least 50% protection against the action of the forcing tool but remains easily seen. A plurality of cover sections may be spaced around the ring circumference and stand out radially from the ring to a distance at which tangents to the outer envelopes of adjacent cover sections do not intersect the tamper indicating ring. In this way, the ring may be provided with 100% protection against the action of the forcing tool but remains easily seen.

Such a cover may be used in conjunction with any of the screw closures having a tamper indicating ring, a frangible link and an anti-backoff pawl as described above.

For a more complete understanding of the invention and some of its further features and advantages, illustrative and non-limiting embodiments are described below with reference to the drawings in which:

FIG. 1 shows a prior art screw cap and tamper-indicating ring, viewed from below, after forced removal using a driving force applied to the tamper-indicating ring;

FIG. 2 shows a prior art screw cap and tamper indicating ring provided with solid, generally triangular anti-backoff pawls;

FIG. 3 is a sectioned view of a first embodiment of a screw cap according to the invention in position on a container neck;

FIG. 4a is an enlarged view from below of a portion of the screw cap shown in FIG. 3;

FIG. 4b illustrates a modification of the screw cap shown in FIG. 4 a;

FIG. 5 provides an enlarged view of one of the flexible anti-backoff pawls shown in FIG. 4 a;

FIG. 6 is a schematic view from above, illustrating screwing of the cap of FIGS. 3-5 onto the container neck;

FIG. 7 corresponds to FIG. 6, but illustrates the action of a cap unscrewing torque;

FIG. 8 shows an alternative form of anti-backoff pawl;

FIG. 9 shows a plan view of a second embodiment of a screw cap according to the invention;

FIG. 10 is an underplan view corresponding to FIG. 9;

FIG. 11 is a scrap view showing portion E of FIG. 10 drawn to a larger scale;

FIG. 12 is a perspective view from below of a third embodiment of a screw cap according to the invention;

FIG. 13 is a partial top plan view corresponding to FIG. 12 and drawn to a larger scale;

FIG. 14a is a scrap sectional view from below, showing the interengagement between rack teeth on a container neck and a tamper indicating ring comprising a fourth embodiment of the invention;

FIG. 14b is a scrap view corresponding to FIG. 14a , showing the tamper indicating ring and an adjacent portion of the screw cap, viewed in the radial direction from inside the cap;

FIG. 15a is a perspective view from below, showing a screw cap which comprises a fifth embodiment of the invention;

FIG. 15b is a perspective view of a container neck and surrounding part of the container body, for co-operation with the screw cap shown in FIG. 15 a;

FIG. 16 is a scrap view showing a portion of a tamper indicating ring and a flexible anti-backoff pawl having a profile comprising a sixth embodiment of the invention;

FIG. 17 is a schematic side view showing a container screw cap in accordance with a seventh embodiment of the invention;

FIG. 18 corresponds to FIG. 4, but shows an embodiment having differently orientated anti-backoff pawls;

FIG. 19 is a perspective view of a screw-threaded plug embodying the present invention;

FIG. 20 is a scrap view from beneath, showing a portion of the rim of the plug of FIG. 19, and

FIG. 21 is a plan view of a container neck configured to receive and be sealed by the plug of FIG. 20.

Referring to FIG. 1, there is shown a container screw closure in the form of a screw cap 10 of known kind, injection moulded in one piece from suitable plastics, such as LDPE. The cap has an internal screw thread 12 for engagement with an external thread formed e.g. on a container neck (not shown). A tamper indicating ring 14 is attached to the remainder of the screw cap 10 by a circumferentially spaced series of frangible links 16. A series of blade-shaped, flexible, anti-backoff pawls 18 have root ends 20 attached to the tamper indicating ring 14. The pawls 18 extend obliquely radially inward from the ring 14, so as to provide freely projecting tips 22. The tips are chamfered to provide a cam surface 24. In normal use, the tip 22 and cam surface 24 co-operate with a series of rack teeth provided on the container (typically distributed about the base of the container neck) so as to provide an anti-backoff action for the tamper indicating ring 14. That is, the anti-backoff pawls 18 flex past the rack teeth so as to allow the screw cap to be screwed onto the container external thread with relatively little mechanical resistance. Hence the shear stress imposed on the frangible links 16 is kept reasonably low, so that they will not break prematurely during cap application and provide a false positive tamper indication. This is the case even if the frangible links 16 are designed to be quite delicate, with a low breaking strength in shear. On the other hand, when the cap 10 is first unscrewed, the pawl tips 22 jam against the rack teeth on the container and strongly resist rotation of the tamper indicating ring 14 on the container in the unscrewing direction. As increasing unscrewing torque continues to be applied to the remainder of the cap 10, the frangible links 16 will break (shear off). The ring 14 thereby becomes detached from the remainder of the cap 10, and this provides the desired tamper indication. Because the frangible links do not have to be robust (due to the low stress generated in them by the anti-backoff pawls 18 during cap application), the ring 14 will detach at a correspondingly low unscrewing torque, making the screw cap 10 easy to open on the first occasion.

However the tamper indicating ring 14 may be circumvented by gripping it and turning it together with the remainder of the cap 10 in the unscrewing direction. As the ring and the rest of the cap are turned substantially in unison, insufficient shear strain is applied to the frangible links 16 to break them. At the same time, a sufficiently high torque may be applied to the ring 14 to violently force the pawls 18 past the container rack teeth. The cap shown in FIG. 1 has been subjected to this treatment. Forcibly overcoming the pawl's jammed condition in this way involves flexing at least the tip 22 in a direction opposite to the normal flexure direction, with a strain many times greater than the designed normal flexural strain. The abnormal strain will usually exceed the cap material's elastic limit and possibly its failure strain, whereby the pawl tips are permanently bent (22 a) and/or broken (22 b). Nevertheless, the anti-backoff pawls 18 may remain sufficiently intact to allow the cap 10 (notably with the frangible links 16 still intact) to be re-applied to the container. The anti-backoff pawls 18 may even still act in the normal way to lock the tamper indicating ring 14 against rotation on the container as the cap 10 is unscrewed again. In that case the frangible links 16 will break and the ring 14 will detach to provide an otherwise normal, but false negative, tamper indication. Without careful inspection of the anti-backoff pawls 18, the end user will have no idea that this has occurred.

FIG. 2 shows another known form of container screw cap 10 made from injection moulded plastics, but which is less vulnerable to circumvention of its tamper indicating ring 14 than the cap as described with reference to FIG. 1. This is because the anti-backoff pawls 18 a have a robust, substantially triangular cross-section in the plane normal to the axis of the tamper indicating ring 14. The anti-backoff pawls 18 a are “solid” in the sense that, compared to the flexible blades 18 of FIG. 1, the acute angled gaps present between the clockwise side of each blade 18 and the adjacent portion of the tamper indicating ring 14 have been substantially “filled in”. The anti-backoff pawls 18 a as shown in FIG. 2 are therefore too stiff to bend contrary to their usual displacement direction and cannot act as rotating levers interposed between the ring 14 and the container circumference. Therefore they cannot be forced out of their jamming contact with the container rack teeth by torque applied to the tamper indicting ring 14 in the unscrewing direction. Instead the torque applying tool tends to slip on the tamper indicating ring 14. This causes relative rotation between the remainder of the cap 10 (which is being driven at the same speed as the tool applying the torque to the ring 14) and the (stationary) ring 14. Hence the frangible links 16 a, 16 b are broken as intended by the cap manufacturer. However, as the cap 10 is screwed onto the container, the rigid anti-backoff pawls 18 a can only move past the container rack teeth by flexing or stretching of the tamper-indicating ring 14 in the radially outward direction. This gives rise to quite a high mechanical resistance to screwing on of the cap. This resistive torque has to be transmitted through the frangible links 16 a, 16 b. Therefore either the frangible links 16 a, 16 b must be made more robust than for the flexible, blade-like anti-backoff pawls 18 of FIG. 1, which leads to an undesirably high initial opening torque necessary to break the frangible links 16 a, 16 b; or there is a danger that the frangible links 16 a, 16 b will break prematurely as the cap is being screwed onto the container for the first time. FIG. 2 shows the cap in a condition where some of the frangible links (16 a) are still intact, while the remainder (16 b) are broken. The tamper indicating ring 14 has an open or unprotected style, in which the frangible links are carried at the corners of lugs 26 projecting radially from the bottom edge of the cap threaded side wall. In contrast, the tamper indicating ring shown in FIG. 1 has a relatively enclosed or protected configuration, in which a radially projecting, circumferential flange 28 at the bottom of the cap side wall overlies and at least partially shields the ring 14, pawls 18 and frangible links 16. Either style or configuration (open or enclosed) may be used in any of the screw caps 10 described in this specification.

FIG. 3 shows a screw closure, in the form of a screw cap 10 embodying the present invention, screwed onto a threaded neck or pouring spout 30 of a container 32 by an internal thread 12. The illustrated cap 10 is formed as a one piece injection moulding of suitable plastics material, but any suitable material and fabrication method may be used. A series of rack teeth 34 are formed on the container, circumferentially distributed around the base of the container neck. These co-operate with a series of flexible anti-backoff pawls (described in more detail below) formed on a tamper indicating ring 14. A series of frangible links 16 separably connect the tamper indicating ring 14 to the remainder of the cap 10. An integrally moulded sealing lip 36 is illustrated depending from the inner surface of the cap top. However this is not essential, and any other suitable sealing element (or none) may be used, depending on the container contents and customer requirements. The illustrated tamper indicating ring 14 and its associated components such as the frangible links 16 have a “closed” configuration, in which they are shielded by an overlying circumferential flange 28; but again this is not essential.

As best seen in FIG. 4a , the frangible links 16 connect the tamper indicating ring 14 to the remainder of the screw cap 10. For example, they may have an L-shape as illustrated, so as to connect the bottom edge of the cap threaded side wall, or the bottom surface of the flange 28, to a top edge region on the interior of the ring 14. Many other forms and configurations of frangible links are known and may be used, e.g. (but not limited to) those shown and described with reference to FIG. 2.

The tamper-indicating ring carries a series of flexible anti-backoff pawls on its inner surface. Some of these (18) may be blade shaped with a freely extending tip 22 and chamfer 24, similar to the pawls 18 shown in FIG. 1. Others 18 b (see also FIG. 5) of the anti-backoff pawls comprise a root end 20 attached to the tamper-indicating ring, and a distal end 22 b co-operable with the rack teeth 34 (FIG. 3) so as to resist relative rotation between the ring 14 and container 32 in the cap unscrewing direction. A flexible link 38 connects the distal end 22 b of the pawl 18 b to the inner surface of the tamper indicating ring 14. If an attempt is made to force the tamper indicating ring 14 to rotate together with the remainder of the screw cap 10 in the unscrewing direction, the flexible link 38 constrains the anti-backoff pawl 18 b and increases its resistance to being bent back on itself contrary to its normal (screwing on) flexing direction, when it encounters the rack tooth 34 on the container neck 30. The forcing tool is therefore more likely to slip on the tamper indicating ring, causing the frangible links 16 to break as intended. At the same time, both the flexible anti-backoff pawl 18 b and the flexible link 38 can be made relatively easily deformable in comparison to a solid tooth on the tamper indicating ring, whereby they do not significantly increase the screwing-on resistance of the cap 10. This imposes lower stresses on the frangible link than e.g. the solid, triangular sectioned anti-backoff pawls 18 a shown in FIG. 2.

As shown in FIG. 4b , in a modification of the embodiment shown in FIGS. 4a and 5, all or a greater number of the anti-backoff pawls may be provided with respective flexible links 38. In other words, there are more pawls 18 b, and fewer or no pawls 18. By adding flexible links 38 to adjacent pawls (18 b), it is ensured that if, with the cap screwed fully on, one of the adjacent pawls 18 b happens to be displaced by a rack tooth, the adjacent pawl 18 b will engage between rack teeth and strongly resist unscrewing of the closure and abnormal bending. Hence any significant forced unscrewing of the closure without also breaking the frangible links 16 and detaching the tamper evidencing ring 14 is prevented.

The flexible link 38 may be made sufficiently thin whereby it readily undergoes buckling collapse. At the same time it is strong enough to prevent the anti-backoff pawl 18 a from being bent back on itself in the opposite direction to its normal deflection direction, when an attempt is made to forcibly unjam it from behind the rack tooth 34 by applying torque to the tamper-indicating ring 14 in the unscrewing direction. The flexible link 38 may be curved or bent along its length in the relaxed state, as illustrated in FIG. 5. This again encourages the flexible link 38 to collapse under lengthwise compression, in order to allow the flexible anti-backoff pawl 18 b to flex and move past the rack tooth 34 in the cap screwing-on direction; again without unduly stressing the frangible link 16 under the drive torque applied to the remainder of the cap 10. For example the flexible link 38 may have a bowed shape between its ends. The centre of the bowed shape may face towards the root end of the flexible anti-backoff pawl (as shown in FIGS. 4 and 5) or away from it.

FIG. 6 is a view from above of the container neck 30 and the associated rack teeth 34 at its base. The tamper indicating ring 14, flexible anti-backoff pawls 18, 18 b and flexible links 38 are indicated schematically in dotted lines, showing their interactions with the rack teeth 34 as the cap is screwed onto the container neck (arrow 40 showing the cap rotation direction). It can be seen that the pawl 18 on the extreme right hand side of the Figure is undeflected, as it is positioned opposite to a rack tooth-free portion of the container neck 30. The remaining pawls 18, 18 b have been cammed radially outward (their tip/distal ends 22/22 b bent towards the ring 14) after their clockwise faces have encountered and ridden up along the sloping, anticlockwise faces of the rack teeth 34. This also causes the flexible links 38 to collapse lengthwise by bowing further outwardly in the anticlockwise direction, as shown.

On unscrewing the cap for the first time (see arrow 42, FIG. 7), the tips/distal ends 22/22 b of the flexible anti-backoff pawls 18/18 b encounter and jam against the radially directed faces of correspondingly positioned rack teeth 34 and begin to bend radially inwards compared to their relaxed state. This tends to straighten the bowed flexible links 38 and allows the flexible anti-backoff pawls 18/18 b to engage more deeply behind their respective rack teeth 34. That is, the pawls 18/18 b deflect radially inward (see straight arrows in FIG. 7) and jam in the corners formed at the bases of the rack teeth radial faces, where these project from the container neck 30.

The flexible link 38 may be joined to the associated flexible anti-backoff pawl 18 b at or near to the distal end 22 b thereof as shown in FIGS. 4-7, so as to provide favourable mechanical advantage and favourable control against excessive bending back of the flexible anti-backoff pawl 18 b. However any joining position may be used in which the flexible link 38 is effective to prevent reverse bending of the pawl 18 b to an extent that it disengages from the rack teeth radial faces, when an attempt is made to subvert the tamper indicating ring 14 by applying unscrewing torque to it.

Although flexible so as to allow movement past the rack tooth in the screwing-on direction, the anti-backoff pawl 18 b can be made sufficiently stiff so as to act as a strut which is resistant to buckling collapse. It thereby provides a high resistance to movement of the tamper indicating ring 14 in the unscrewing direction, when the distal end 22 b is wedged against the container rack tooth 34. The flexible anti-backoff pawl may be flexibly attached to the tamper indicating ring, as shown in FIG. 8. Here a thinned section 44 c, or even a living hinge, is shown at or near to the root end of the anti-backoff pawl 18 c. Similarly, the flexible link 38 c may be provided with one or more thinned sections 46 along its length, to form more flexible sections or living hinges, for the same purpose. The flexible link need not be bowed. The centre flexible section 46 is shown to be out of alignment with the end flexible sections, to encourage the flexible link to bend and collapse predictably in the outward direction away from the anti-backoff pawl 18 c; although inwardly collapsing arrangements and arrangements with unpredictable directions of collapse may also be used.

FIGS. 9 and 10 respectively are top and underplan views of a further container screw cap embodying the present invention. The screw cap 10 is similar to that shown in FIG. 2, in that an open or unprotected style of tamper indicating ring 14 is attached to the outer corners of six projecting lugs 26 by frangible links (not clearly visible). Finger grip ribs 48 extend from side edges of each lug, up the side wall of the cap and into a curved transition region between the cap side wall and top wall.

An inner surface of the tamper indicating ring 14 is formed with anti-backoff pawls 18 d, one of which appears in region E of FIG. 10; region E being drawn to a larger scale in FIG. 11. Near to its distal end 22 d, the pawl 18 d is attached to the ring 14 inner wall by an inwardly curving strut 38 d. This provides a flexible link between the pawl distal end and the tamper indicating ring 14. As the pawl 18 d rides up the sloping faces of the rack teeth 34, the distal end 22 d is able to flex radially outwardly so as to move closer to the ring 14. In doing so, the strut 38 d also flexes to a more curved shape. In this condition, the majority of the pawl 18 d has a convex curvature at its radially inner surface, with a degree of inflection towards its root end 20 d. The overall result is that as it passes the rack teeth 34, the flexible anti-backoff pawl 18 d is deformed to adopt a lower profile (as compared not only to its unstressed profile, but also as compared to a solid pawl such as 18 a of FIG. 2). It therefore offers less mechanical resistance and does not impose as high stresses on the frangible links, with the advantages for reliability and ease of use as discussed above. The radially inner face of the pawl 18 d is angled in the cap axial direction, sloping radially inwardly from bottom to top, as can be seen in FIG. 11. This also helps the pawls 18 d to ride smoothly past the rack teeth 34 as the cap 10 is screwed down onto the container neck.

As the cap 10 is torqued in the unscrewing direction, the pawl distal ends 22 d jam against the radially directed faces of the rack teeth 34. The strut 38 d may straighten slightly e.g. if torque is applied directly to the tamper indicating ring 14 in an attempt to circumvent it. This allows the pawl distal ends 22 d to flex radially inward to jam in the corners formed at the bases of the rack teeth radial faces, where these project from the container neck 30; but the struts 38 d constrain any further flexing and do not allow the pawl distal ends 22 d to be forcibly bent past the rack teeth 34. The torque applying tool therefore slips relative to the tamper indicating ring 14 and breakage of the frangible links is assured.

FIGS. 12 and 13 show a modification of the screw cap shown in FIGS. 9-11, in which the tamper indicating ring is interrupted to form segments 14 e. Ends of adjacent segments are connected in series by the flexible anti-backoff pawl 18 e and the flexible link/curved strut 38 e. In other words, in FIGS. 12 and 13 the ring/strut/pawl structure is as if the parts of the ring 14 in FIGS. 9-11 between each pawl 18 d and its corresponding strut 18 d have been removed. The resulting segmented ring structure is able to expand circumferentially, which allows the pawls 18 d to ride past the rack teeth 34 even more easily in the screwing-on direction. In the unscrewing direction, any torque applying tool used to grip the ring segments:

a) has less surface area to gain a purchase on, and

b) tends to constrain the ring segments against radial expansion, so that the struts 38 e, pawls 18 e and pawl distal ends 22 e behave in the same way as the components 38 d, 18 d and 22 d described with reference to FIGS. 9-11, insofar as is material. The frangible links 16 are also more clearly visible in FIG. 12; those used in the embodiment of FIGS. 9-11 may be similar.

FIG. 14a shows portions of another embodiment of the invention viewed from below the container neck and screw cap. Here the tamper-indicating ring 14 is equipped with anti-backoff pawls 18 shaped as blades, e.g. similar to those used in the screw cap 10 shown in FIG. 1. A guard stop 50 is rigidly attached to the tamper-indicating ring adjacent to at least one of (and optionally all or a substantial proportion of) the flexible anti-backoff pawls 18 f ₁. The guard stop 50 is positioned to contact and support the flexible anti-backoff pawl 18 f ₁ so as to resist bending thereof driven by contact with the rack tooth 34 f ₁, as the cap is torqued in the unscrewing direction. Thus the guard stop 50 is positioned in the obtuse angled space anticlockwise of the pawl 18 f ₁ as viewed in FIG. 14. The gap 52 between the guard stop 50 and the pawl 18 f ₁ may be kept as small as is practicable according to the moulding process, and in any event should be kept sufficiently small to avoid excessive deformation of the pawl 18 f ₁ to the point at which it can be forced past the rack tooth 34 f ₁ in the cap unscrewing direction. The gap 54 between the other side of the guard stop 50 and the next pawl 18 f ₂ (if such a pawl is present), must be large enough to accommodate the normal flexing of the pawl 18 f ₂ as the cap is screwed onto the container. That is to say, the gap 54 is necessary to allow the pawl 18 f ₂ to clear the rack tooth 34 f ₂.

Referring to FIG. 14b , it can be seen that the illustrated guard stop 50 is thinner than the width (height) of the tamper indicating ring 14 in the cap axial direction and positioned adjacent to the upper edge 56 of the tamper indicating ring 14, so as to provide a clearance space 58 below the guard stop 50 for passage of the rack teeth 34 f ₁, 34 f ₂ formed on the container.

Another embodiment of the invention is shown in FIGS. 15a and 15b . The guard stop 72 shown in FIG. 15a comprises an extension of the internally threaded, annular cap side wall 70, beyond and below the radially projecting, circumferential flange 28. The guard stop 72 in this form is therefore annular; lying adjacent to, radially inward of, and slightly spaced from, the inwardly projecting ends of the anti-backoff pawls 18. For containers having ratchet teeth formed directly on their necks, this would be the position occupied by the ratchet teeth when the screw cap 10 is fully tightened onto the container neck. Therefore, as shown in FIG. 15b , rather than being formed directly on the container neck 30, the ratchet teeth 74 are formed upstanding from a portion of the container body, concentric with the container neck 30, to define an annular gap 76. When the cap 10 is screwed onto the container neck 30, the guard stop 72 occupies the annular gap 76. The annular side wall extension 72 (guard stop) and the container neck 30 in this position form a relatively rigid structure which constrains abnormal (backwards) bending of the anti-backoff pawls 18. In a variant (not shown), the annular side wall extension forming the guard stop 72 does not extend downwards across the full width of the anti-backoff pawls 18, but instead terminates part-way across their widths. The ratchet teeth can then be provided on the container neck in the standard way, lying immediately beneath the guard stop 70 when the screw cap 10 is fully tightened onto the container neck 30. In this configuration, the lower parts of the pawls 18 engage the rack teeth and the upper parts of the pawls lie opposite to the guard stop 70. In this variant, the distance between the tips of the pawls 18 and the guard stop 70 is kept small or may even be eliminated, within the constraints allowed by moulding considerations.

Another embodiment of the invention is shown in FIG. 16, in which the distal end 22 g of the flexible anti-backoff pawl 18 g has an enlarged cross-section whereby it is selectively stiffened to resist bending. In the illustrative example shown, the flexible anti-backoff pawl 18 g progressively increases in cross-sectional thickness along its length from the root end 20 towards the distal end 22 g. The flexible anti-backoff pawl comprises a thinned section 44 g by which it is flexibly attached to the tamper indicating ring 14, for example, at or near to its root end 20. Like the solid pawls 18 a of the cap shown in FIG. 2, the pawl 18 g is too stiff in the critical areas most vulnerable to excessive reverse bending, i.e. at and towards the distal end 22 g, to bend contrary to its usual displacement direction. Therefore the pawl 18 g cannot be forced out of jamming contact with the adjacent container rack tooth 34 by torque applied to the tamper indicting ring 14 in the unscrewing direction. However, rather than being rigidly attached to the tamper indicating ring 14, the anti-backoff pawl 18 g can flex relative to the tamper indicating ring 14 in a manner similar to the blade-shaped pawls 18 of FIG. 1, in order to pass the rack teeth in the screwing-on direction. (Except that the flexing is concentrated mainly at the thinned section 44 g, rather than taking place along the entire length of the pawl 18 g). The result is that the pawl 18 g presents less mechanical resistance to screwing on of the cap than do the solidly attached pawls 18 a of FIG. 2; with the corresponding advantages for reliability and ease of use as discussed above. A sufficiently large gap 64 is provided between the outer, clockwise side of the pawl 18 g and the adjacent inner surface of the tamper indicating ring 14, so as to allow flexing of the pawl 18 g so that it may pass the rack teeth 34 in the screwing on direction, without substantial outward flexing of the ring 14 itself.

The distal end 22 g of the flexible anti-backoff pawl 18 g may comprise a first, substantially flat face 60 configured for substantially face-to-face engagement with the generally radially extending face of the rack tooth 34 on the container, and a second, curved face 62 for engagement with the container (on the part of the container onto which the cap is being screwed, typically the neck 30), adjacent to the rack tooth 34. This configuration helps to reduce stress concentrations arising under axial compressive loading of the pawl 18 g when abnormally high backoff forces are imposed on the pawl 18 g e.g. as a result of attempts to circumvent the tamper indicating ring 14. It also helps to further stabilise the pawl 18 g against abnormal reverse bending. The widely spaced edges of the contact region between the face 60 of the pawl 18 g and the rack tooth radial flank, and the similarly widely spaced edges of the contact region between the face 62 and the container side wall at the base of the rack tooth radial flank, stabilise the pawl 18 g against pivoting on the rack tooth and/or container wall when subjected to loading in the unscrewing direction. The pawl 18 g is therefore less liable to break, permanently deform or be forced past the rack tooth 34 in the unscrewing direction under such abnormal loading, than the pawls shown in FIG. 1.

FIG. 17 shows a further embodiment of the invention in which the screw cap 10 comprises a cover overlying at least a part of the outwardly facing, outer circumference of the tamper indicating ring 14. The cover thereby prevents access to the full circumferential extent of the tamper indicating ring, making driving engagement with the tamper indicating ring for forced bending and disengagement of the flexible anti-backoff pawls more difficult.

The cover comprises a number of sections 66, between which the tamper indicating ring 14 is left exposed and visible, so that its presence or absence can readily be seen when the cap 10 is screwed onto the container. Each section 66 may stand out radially from the tamper indicating ring 14 to an extent that for example 50% of the ring lies between tangents to the ring and the outer portion of the envelope of each cover section. In this way, the ring 14 may be provided with 50% protection against the action of the forcing tool, but remains easily seen. Other extents of coverage or protection are also possible. The bars 68 in FIG. 17 schematically indicate the circumferential extent of the protection afforded by the cover sections 66 and the “shadows” at their ends. The plurality of cover sections 66 may be spaced around the ring 14 circumference and stand out radially from the ring to a distance at which tangents to the outer envelopes of adjacent cover sections do not intersect the tamper indicating ring. In this way, the ring may be provided with 100% protection against the action of the forcing tool but remains easily seen. The various cover sections 66 have open bottoms (thereby for example having a generally L-shaped radial cross-section). This allows the remainder of the cap 10 to be lifted away while leaving the tamper indicating ring 14 behind on the container when the cap 10 is unscrewed for the first time.

FIG. 18 illustrates a screw cap generally similar to that shown in FIGS. 3-7, except that the orientation of the anti-backoff pawls 18, 18 h of FIG. 18 differs from the orientation of those 18, 18 b of FIGS. 3-7, and the tamper indicating ring 14 h is correspondingly modified. Thus a tamper indicating ring 14 h of generally L-shaped cross section is attached to the remainder of the screw cap 10 by a circumferentially spaced series of frangible links 16 h. A series of blade-shaped, flexible, anti-backoff pawls 18, 18 h have root ends 20 attached to the tamper indicating ring 14 h. The pawls 18, 18 h extend obliquely axially downward from the ring 14 h, so as to provide freely projecting tips 22 and distal ends 22 h respectively. The tips are faceted or chamfered to provide a cam surface 24. In normal use, the tips 22/distal ends 22 h and the cam surfaces 24 co-operate with a series of rack teeth provided on the container (typically distributed about the base of the container neck) so as to provide an anti-backoff action for the tamper indicating ring 14. The rack teeth are still typically distributed circumferentially about the base of a neck or pouring spout of the container, but are correspondingly re-orientated compared to those shown and described in FIGS. 3, 6 and 7, so that they extend axially relative to the screw cap, with their locking flanks extending axially upward from root to tip, rather than radially outwardly from root to tip. The rack teeth are therefore arranged below the anti-backoff pawls 18, 18 h, about pitch circles of the same diameter, rather than being arranged generally radially inward of the anti-backoff pawls 18, 18 b as shown in FIGS. 3, 6 and 7. The pawls 18, 18 h of FIG. 18 therefore flex in a generally axial direction, instead of in a generally radial direction as is the case for the pawls 18, 18 h of FIGS. 3-7. The operation of the screw cap shown in FIG. 18 is otherwise generally similar to the screw cap shown and described with reference to FIGS. 3-7. The embodiments shown in FIGS. 9-17 may be correspondingly modified, as regards the directions of projection and of deflection of their anti-backoff pawls (with appropriate corresponding modification of the rack teeth orientation, and of the position and orientation of the guard stop 50 in the case of FIGS. 14a and 14b . That is, the modified guard stop 50 is thinner than the width of the tamper indicating ring 14 h in the radial direction and positioned adjacent to a radially inner or outer edge of the ring to allow for passage of the rack teeth. A position adjacent to the ring outer edge is preferred, but not essential). The relative numbers of the pawls 18, 18 h may be varied as desired, similarly to the pawls described with reference to FIGS. 4a and 4 b.

Although the various embodiments particularly described above concern container screw caps, the invention may be more broadly applied to any suitable screw closure for a container, such as a screw plug provided with a tamper-indicating ring. Such a plug, and a corresponding container neck to be closed by the plug, are shown by way of non-limiting example in FIGS. 19-21.

As shown in FIG. 19, the plug 80 comprises a generally flat, circular top 88, having a projecting, frangibly detachable rim 82, delineated from the remainder of the plug by through-going arcuate slots 84. Bridging pieces 86 are defined between the ends of adjacent slots 84, by which the rim 82 remains frangibly attached to the rest of the cap top 88. One of the bridging pieces 87 may have an s-shaped or otherwise elongated configuration, to form a flexible strap by which the rim 82 remains attached to the rest of the plug 80 after the other bridging pieces 86 have been broken.

As best shown in FIG. 20, the underside of the rim 82, radially outward of the slots 84 and bridging pieces 86, is provided with a series of downwardly and anti-clockwise extending blade-like pawls 18, whose tips are chamfered to provide cam surfaces 24. Alternating with the pawls 18 is a second series of pawls 18 i, whose tips are connected to the underside of the rim 82 by respective flexible links 38 i. These act in essentially the same way as the flexible links attached to the pawls 18 h in FIG. 18, and in essentially the same way as the flexible links 38 shown in FIGS. 4a, 4b and 5; to constrain the pawls 18 i against abnormal reverse bending. As with FIGS. 4a and 4b , the relative numbers of pawls 18, 18 i may be varied as desired.

The pawls 18, 18 i co-operate with ratchet teeth 74 i upstanding from an otherwise generally flat, annular surface 94 at the upper end of the container neck 90, surrounding the container filling, emptying or venting opening 92, as shown in FIG. 21. The plug 80 has a threaded shank 102 which can be screwed into an internal thread in the neck opening 92. The ratchet teeth 74 i each have an inclined face 96, and a vertical, clockwise-directed face 98. The annular surface 94 is surrounded by an upstanding protective ring 100, which restricts access to the pawls 18, 18 i.

The plug rim 82 may be provided with a chamfered outer edge 104 at its undersurface, which lies above the neck ring 100 when the plug 80 is received in the container neck 90. The chamfered outer edge 104 and neck ring 100 may thereby define a tapered, radially outwardly directed groove which snap-fittingly receives a protective overcap (not shown). When the tamper-indicating plug rim 82 has been broken off, it is no longer possible to re-fit the overcap. When fitted, the overcap covers the top of the plug 80, including an aperture 106 for a wrench or similar drive tool, and helps to prevent the ingress and accumulation of dust, dirt, moisture and other contaminants. An annular sealing washer seat 108 extends radially outwards from the upper end of the plug shank 102, on the plug top undersurface, inboard of the slots 84 and bridging pieces 86. 

1. A screw closure for a container, the screw closure comprising: a tamper indicating ring; a frangible link connecting the ring to the remainder of the screw closure; a flexible anti-backoff pawl having: a root end attached to the tamper-indicating ring, and a flexibly movable distal end co-operable with a rack tooth formed on the container so as to allow in normal operation relative rotation between the ring and container in the closure screwing-on or -in direction by flexing of the pawl and so as to jam against a rack tooth and resist relative rotation between the ring and container in the closure unscrewing direction; characterised in that the screw closure further comprises a flexible link connecting the flexible anti-backoff pawl and the tamper indicating ring; the flexible link being configured to undergo buckling collapse during screwing on or in of the closure; the flexible link being of a strength to remain unbroken when a forcing tool is used to attempt to move the ring and the remainder of the screw closure in unison in the unscrewing direction, whereby the anti-backoff pawl is constrained by the flexible link against being bent abnormally when the anti-backoff pawl encounters the rack tooth, whereby the forcing tool slips on the ring and causes the frangible link to break.
 2. The screw closure of claim 1, in which the flexible link is provided with one or more sections along its length which are thinner than other sections along its length, to form more flexible sections.
 3. The screw closure of any preceding claim, in which the flexible link comprises a curve or bend along its length in the relaxed state.
 4. The screw closure of any preceding claim in which, when the pawl is in the relaxed or normal flexed positions, the flexible link comprises a bowed shape.
 5. The screw closure of claim 4, in which the convex side of the bowed shape faces away from the root end of the flexible anti-backoff pawl.
 6. The screw closure of any preceding claim, in which the flexible link is joined to the flexible anti-backoff pawl at or near to the distal end thereof.
 7. The screw closure of any preceding claim, in which the flexible anti-backoff pawl is flexibly attached to the tamper indicating ring.
 8. The screw closure of any preceding claim, in which the flexible anti-backoff pawl comprises a section at or near to its root end which is thinner than a part of the anti-backoff pawl more distant from the root end.
 9. The screw closure of any preceding claim, in which the tamper indicating ring is interrupted to form segments.
 10. A screw closure for a container, the screw closure comprising: a tamper indicating ring; a frangible link connecting the ring to the remainder of the screw closure; a flexible anti-backoff pawl having: a root end attached to the tamper-indicating ring, and a distal end co-operable with a rack tooth formed on the container so as to allow relative rotation between the ring and container in the closure screwing-on or -in direction and so as to resist relative rotation between the ring and container in the closure unscrewing direction; characterised in that a guard stop is formed on or attached to the screw closure adjacent to the flexible anti-backoff pawl and positioned to contact and support the flexible anti-backoff pawl so as to resist bending thereof driven by contact with the rack tooth as the closure is torqued in the unscrewing direction.
 11. The screw closure of claim 10, in which the guard stop is thinner than the width of the tamper indicating ring in the closure axial direction and formed on or attached to the tamper indicating ring adjacent to an upper edge thereof; or in which the guard stop is thinner than the width of the tamper indicating ring in the closure radial direction and formed on or attached to the tamper indicating ring adjacent an inner or outer edge thereof.
 12. The screw closure of claim 10, in which the guard stop comprises an extension of an annular side wall of the screw closure positioned opposite to the distal end of the anti-backoff pawl.
 13. A screw closure for a container, the screw closure comprising: a tamper indicating ring; a frangible link connecting the ring to the remainder of the screw closure; a flexible anti-backoff pawl having: a root end attached to the tamper-indicating ring, and a distal end co-operable with a rack tooth formed on the container so as to allow relative rotation between the ring and container in the closure screwing-on or -in direction and so as to resist relative rotation between the ring and container in the closure unscrewing direction; characterised in that the distal end of the flexible anti-backoff pawl has an enlarged cross-section compared to the remainder of the anti-backoff pawl, whereby the anti-backoff pawl is stiffened to resist bending.
 14. The screw closure of claim 13, in which the flexible anti-backoff pawl progressively increases in cross-sectional thickness along its length from the root end towards the distal end.
 15. The screw closure of claim 13 or 14, in which the flexible anti-backoff pawl is flexibly attached to the tamper indicating ring.
 16. The screw closure of claim 13, 14 or 15, in which the flexible anti-backoff pawl comprises a thinned section, at or near to its root end.
 17. The screw closure of claim 13, 14, 15 or 16, in which the distal end of the flexible anti-backoff pawl comprises a first, substantially flat face configured for substantially face-to-face engagement with a generally radially extending or generally vertically extending face of the rack tooth on the container.
 18. The screw closure of claim 17, in which the distal end of the flexible anti-backoff pawl comprises a second face for engagement with the container adjacent to the rack tooth.
 19. The screw closure of any of claims 13-18 and any of claims 1-9.
 20. A screw closure for a container, the screw closure comprising: a tamper indicating ring; a frangible link connecting the ring to the remainder of the screw closure; and a flexible anti-backoff pawl attached to the tamper indicating ring; characterised in that the screw closure further comprises a cover overlying at least a part of the outer, radially outwardly facing, circumference of the tamper indicating ring.
 21. The screw closure of claim 20, in which the cover leaves at least one section of the tamper indicating ring exposed and visible.
 22. The screw closure of claim 20 or 21, in which the cover comprises one or more sections.
 23. The screw closure of claim 22, in which the or each section stands out radially from the tamper indicating ring to an extent that at least 50% of the ring lies between tangents to the ring and the outer portion of the envelope of the or each cover section.
 24. The screw closure of claim 22, in which a plurality of cover sections are spaced around the ring circumference and stand out radially from the ring to a distance at which tangents to the outer envelopes of adjacent cover sections do not intersect the tamper indicating ring.
 25. The screw closure of any of claims 20-24 and any of claims 1-19. 